We've had a lot of intense and diabolical witches try and take over New Orleans during the last two seasons, but The Originals' Dahlia (last name TBD, we've never learned Esther's maiden name) seems to be the biggest threat to the Mikaelsons yet. She blew into New Orleans after much threatening and scary foreshadowing from Freya. The same Freya who has spent hours trying to convince her long lost siblings that she hates Dahlia, is trying to run from her, because basically, she is firmly on Team-Kill-Dahlia-She's-The-Worst. She's been pretty convincing with her hatred and her fear, and I think it's very likely that she hates Dahlia every bit as much as she says. However, I think it is much more likely that Freya and Dahlia are working together to get Hope, even if Freya isn't thrilled about it.
Dahlia has been channeling Freya's powers for centuries now, keeping her in a sort of witch indentured servant agreement that her mother made long ago. Dahlia needs to collect Hope as the latest payment for all first born Mikaelson babies, to honor the spell and keep her powers up. (Sidenote: why is it always the first born? There's never any love for the middle child.) Hope is crazy well-guarded in New Orleans, so my guess is that Dahlia is using Freya to infiltrate Freya's siblings, earn their trust, and gain access to Hope. Once Freya has given Hope to Dahlia, she has promised to free Dahlia from her service now that she has a new spell power-pack in Hope.
Sure, it's messed up. But if you had been stuck living someone else's life for 1,000 years and was never able to live, age, or love normally, you'd probably be willing to do some messed up things, too. It follows with what we've seen of Freya so far. She's made inroads with Rebekah and Elijah from the get-go, hell bent on having them trust her. Then, she very conveniently needs to see inside Klaus and Elijah's minds to save Rebekah — something I am not sold on being necessary. Finally, she has a change of heart and decides she needs to isolate Klaus entirely? So, so suspicious Freya. But it's in line with someone playing a larger con, trying to take advantage of her more trusting siblings.
With any luck, someone will catch wise to her plans soon. Dahlia is enough to scare even Mikael — and Freya never once swore "always and forever."
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